I always feel guilty wasting perfectly good papers

Lol, me too. But if you can, watch some older artists on Youtube: more expensive papers just make certain things easier. Cheap papers make the same thing harder -- NOT impossible.

That's how cheap paper builds your skill: you're forced to pay sharp attention to the dampness of the paper, the exact sheen of it in the light, to know when it's ready for a blend or a scrape (where you scratch into wet paper to either create dark lines if it's wetter or remove paint if it's drier), etc.

I don't know if you're in the US but the paper I like most for watercolor is also one of the least expensive: Canson XL watercolor paper. It's a nice, heavy 300g that takes a good bit of abuse from scratching, scrubbing, lifting, and pretty much anything else I want to do. ALL watercolor paper will buckle, but the heavier weights keep it to a minimum, and 300g is an all-around great weight (heavier weights are professional use and tend to get really expensive). Arteza I have also heard good things about, inexpensive and good, but Canson's usually at the Walmart so I can just get some whenever I'm there and avoid Amazon, lol.

For painting boards, I save and use the clean tops of pizza boxes, lol. Any clean stiff cardboard will do, turn it over and use the plain backside so there's no visual confusion, and cheap masking tape or washi to hold it down if you don't feel like freestyling it, lol. For the copy paper practice I wouldn't even worry about that, any tape you want to use. (You can also use scotch tape if you weaken the hold on it a bit by taping across your clothes before you put it on your paper, if you're in a pinch and need a removable tape.)

Your watercolors came in a tray; that's also a paint mixing tray. Take the paint tubes out while you're painting and then rinse it afterwards to use for tube storage again. And use any old plastic container for water, etc. Now you're all set!

Sure. When your watercolor painting dries, it's perfect as-is, and you need do nothing more, especially if you know it's going in a drawer or a frame or even in a pile of other watercolors in the corner, lol. However, it will react with any kind of moisture, because that's what watercolors do. If your watercolor work gets wet or is in a really high humidity area, touching it or allowing it to touch anything else will smear the paint.

So if you have some work you really like and want to protect when it's done, you can use a surface fixative (usually a spray) to protect it from moisture. But greeting cards don't usually see a lot of water or humidity, so it's up to you. It will not be obvious either way: fixatives are completely invisible to the eye until water is involved and then it's pretty obvious what's been sealed and what hasn't been, lol.

I have a couple watercolor paintings hanging in my bathroom, for example, but it's not a problem because I sealed them with a surface fixative as well as Dorland's (a wax made specifically for artwork) and they're behind glass, so the occasionally high humidity in there doesn't matter at all. But that's kind of an extreme case.

Fixatives are pretty cheap and a little goes a long way, but it's up to you as to whether you want to use one. If you do, just remember that the fixative is the very LAST thing you put on, because you usually can't work the painting after you've done it. (There are "workable" fixatives that you can put on and then still add to your work, but just how "workable" they are really depends on the specific fixative and the specific media you're using, so don't count on it.)

You said you are making cards; IMO you would be fine leaving your work as-is or sealing them. Honestly either way is fine, your choice. But if you do use one, make the fixative the very LAST thing you do to your art, at least until you have a chance to experiment with any fixative you buy. And of course, do any ironing prior. Hope this helps.